Abstract

Central Oregon (northwestern USA), where northern Basin and Range extension diminishes in magnitude across the High Lava Plains, exhibits widespread extensional faulting and Quaternary volcanism, yet the relations between the processes are complex and chronology is poorly constrained. Here we use cosmogenic 3He exposure dating of basalt lava flows to quantify the timing of normal faulting and emplacement of a lava field on the margins of pluvial Fort Rock Lake. The northwest-trending Christmas Valley fault system displaces High Lava Plains volcanic rocks, forming an ∼3-km-wide graben that transects the eastern Fort Rock Basin. A portion of the western edge of the graben is marked by a normal fault displaying flexural shear folding with a prominent vertical hinge crack, called Crack in the Ground. Lava flows of the Four Craters Lava Field flowed into this crack. Exposure dating of the Four Craters Lava Field, on the eastern flank of the older Green Mountain shield volcano, indicates an emplacement age of 14 ± 1 ka. We dated Green Mountain basalt exposed in the walls of the crack (the fault wall), which also yielded exposure ages of 14 ± 1 ka. The similar ages suggest that substantial crack opening occurred at about the same time the Four Craters lava was emplaced. These data indicate a period of synchronous normal faulting and dike-fed cinder cone activity emanating from a fault-parallel fissure ∼2 km northeast of the crack ca. 14 ka, with minimal displacement since.